The present invention is directed to a mixing system and method of mixing for use in blending powders. The invention is especially suitable for blending coating powders, particularly where one component is a liquid-like, resinous base composition and another is a tinting agent or other additive in finely ground, dry powder form. Such blending results in a coating powder of uniform, homogenous properties, with color being an especially important property.
Various systems and methods for mixing powders are well known in the industry.
U.S. patent application Ser. No. 10/102,216 of Steven M. Ladatto, filed Mar. 20, 2002, entitled xe2x80x9cCoating Powder Compositions and Methodsxe2x80x9d is directed to providing a liquid-like, resinous coating powder base composition having a melt viscosity of from about 2 Paxc2x7s to about 85 Paxc2x7s. Such base compositions typically range in particle size from about 20 to about 200 microns. These base compositions can be produced in large batches, and then smaller portions of such batches can be mixed with various tinting agents and/or other additives to obtain a small batch of a desired coating powder having a desired color or other property. Typically, tinting agents are in a finely ground state having a particle size of 5 microns or less, or preferably, 3 microns or less to maximize total surface area per unit mass. Other additives may include particle sizes on the order of 10 microns or less and may range upwardly to about 35 microns or more.
The above-mentioned patent application discloses mixing of the respective powders into a final coating powder using a conventional mixing vessel that imparts shear to the materials, thereby producing the desired uniform, homogeneous coating powder. A suitable high intensity mixer mentioned in the patent application is commercially available from Henschel. Henschel mixers have a mixing blade disposed near the bottom of the mixing container. Mixing start-up occurs while the material to be mixed surrounds the mixing blade, thereby incurring the need for more powder during startup than if the mixer were to be started under no-load conditions. Such blades typically may comprise four mixing blades located at the bottom of the mixing container. The blades have nominal pitch (less than about 5xc2x0 from horizontal). The four blades are in sets of two balanced, oppositely disposed blades. Each set of two blades is located above the other set. Thus, each set of two blades is in separate planes. Henschel mixers of the type described above are designed to mix and reduce the size of raw materials rather than to mix powders.
Other potentially useful mixers are available commercially from Mixaco and are further described in The Science of Powder Coating Applications, Volume 2, pages 259-261, published by SITA Technology, London, England, 1994. Such mixers are described to operate by loading premix materials into a cone-bottomed container, clamping a mixing lid containing a mixing blade to the top of the container, inverting the container/mixing lid assembly through 180xc2x0, commencing mixing, stopping mixing, and moving the container/mixing lid back through 180xc2x0, and then discharging the mixed product. These mixers typically utilize a two-blade set mixing assembly having the blades disposed generally perpendicular (90xc2x0) to the mixing container bottom.
The mixing system and method of the invention possess several significant advantages when contrasted to the above-described mixing systems and methods. First of all, while a Henschel high intensity mixer could be used to produce a suitable product, when used to mix the liquid-like, low melt viscosity resinous base powders mentioned above, there may be a tendency to heat and fuse such powders on the mixer blade. Fused powder on the blades can subsequently flake off and contaminate, or introduce non-uniformity in, the finished, mixed powder. The present invention does not encounter the above-mentioned fusion problem because its blade design promotes rapid, efficient mixing and the resultant short mixing times minimize the amount of heat buildup due to friction.
Secondly, Henschel mixers are not inverted during use; thereby its mixing blades are not started under no-load conditions. On the other hand, the method of the present invention involves no-load starting and thus capitalizes on the inversion of the mixing container. This advantage is especially beneficial in the mixing of powders because the blade is in motion as the powders gradually make contact with the blade. As the container reaches 90xc2x0 on its way to being inverted 180xc2x0, the contents of the container begin to fall onto the blade. This method has at least two advantages. First, a gradual load on the motor which permits the use of a smaller motor when contrasted to using a design which has the entire weight of the material at rest on the blade when the blade rotation is started. Secondly, variable mixing dynamics occur between 90xc2x0 and 180xc2x0 rotation. This mixing dynamic permits the powder to move in various directions on the way to its 180xc2x0 inversion position which then incurs a predictable mixing dynamic.
As stated above, the above-described Mixaco mixers appear not to be designed to mix powders in the manner of the present invention. This conclusion is apparent when differences between blade design and rotation speed are considered.
Neither the Henschel nor the Mixaco mixers discussed above employ the rotary blade mixing assembly and blade design of the present invention. Such blade design is adapted for use in mixing liquid-like resinous powders, especially when one powder component is a resinous powder having a viscosity from about 2 Paxc2x7s to about 85 Paxc2x7s and a particle size ranging from about 20 to about 200 microns.